[GRASS-user] Some questions about DEMs

[Maciej Sieczka]

W dniu 09.07.2010 12:29, David Townshend pisze:

> I have a couple of questions about DEMs, and how they work in GRASS. My
> application is hydrological and hydraulic analysis, and at the moment I'm
> investigating using r.sim.water to generate floodlines.
>
> Usually, for this sort of analysis, I would have 5m contours for the
> catchment and a detailed survey (i.e. spot heights, sometimes with
> triangles) for sections of river. So my first question is, what is the best
> way to generate a DEM from this information? I know that I can use
> r.surf.contour to create a DEM from the contours, and I'd assume that
> something like v.surf.rst would be best for the points, but I need a single
> DEM for the analysis.

For heterogenous input data I'd try the nn algorithm in nnbathy, through 
r.surf.nnbathy (http://grass.osgeo.org/wiki/Addons#r.surf.nnbathy).

> The second problem is that I don't need a highly detailed DEM for the
> catchment, I only need detail along the watercourses - i.e. where I have the
> detailed survey. If I create two DEMs at different resolutions, can I patch
> them without losing detail?

You can patch 2 rasters of different resolution, but not to loose 
information, the output raster map needs to be of the resolution of the 
more detailed input raster map. This will result in a flat squares + 
smooth valleys surface.

> My third question is about creating a mask. If I'm working along the coast,
> or if I'm creating a detailed DEM along the watercourse, I only want raster
> data where there is base data (i.e. contours or points). Is there a way to
> quickly generate a mask for this?

v.hull + v.to.rast?

Maciek

-- 
Maciej Sieczka
http://www.sieczka.org